This invention relates to opto-acoustic absorption detectors commonly referred to as spectrophones, and more particularly to a method and apparatus for measuring the concentrations of absorbing gases in a sample cell using opto-acoustic detection of absorption.
There is an increasing interest in detecting trace amounts (approaching 0.01 parts per billion) of atmospheric pollutant gases in an air sample. However, the residual background signal, such as that produced by the heating of the cell windows, limits the sensitivity of opto-acoustic absorption detectors. Terrence F. Deaton, David A. Depatie and Thomas W. Walker report their efforts to overcome this problem of sensitivity with a differential cell in a paper titled "Absorption Coefficient Measurements of Nitrous Oxide and Methane at DF Laser Wavelengths," Applied Physics Letters, Vol. 25, No. 6, 300-303 (1975). Two identical cells were used in tandem. Both were first filled with non-absorbing gas, and the pressure differential signal between the cells was then minimized while the laser beam was transmitted through both cells to "zero" the instrument. Absorption measurements were then made by filling one cell with a sample. The pressure differential signal between the two cells represents the absorption coefficient of the test gas. Residual pressures which are present as background signals due to absorption by the windows of the cell is effectively balanced out by this double cell arrangement, but it has the problem that extreme care must be exercised in filling both cells to the same pressure during the balancing procedure and during the measuring procedure. Still another approach to the problem referred to, but not discussed in their paper, is the development of a "windowless resonant cell" by David A. Depatie. The problem with that approach is said in that paper to be that "the pickup of ambient acoustic noise increases."